When fluid is extracted from a subsea oil and gas well, the fluid produced is normally conveyed from the wellbore to a production facility, for example a Floating Production Storage and Offloading vessel (FPSO) located on the surface of the ocean (also known as topside), via various items of subsea equipment. Typically, the subsea equipment includes a Christmas Tree, which is an assembly of valves, spools and pressure gauges fitted to the wellhead to control production.
During the production of fluid from the subsea oil and gas well, the produced fluid is transported through pipelines which may comprise two parts, (i) a flowline, which is the portion of the pipeline located on the seabed and which transports the produced fluids from the wellhead either to a manifold or directly to the topside (for example when using a SCR (Steel Catenary Riser)); and (ii) a riser, which is the portion of the pipeline which extends from the seabed to the production facility. In order to gather the production of several wells located in the same area and convey the produced fluid through one flowline a manifold may be used. The manifold is normally connected to the flowline on the one side and to the Christmas Tree on the other side, via a jumper (ie a flexible or rigid section of pipe, also known as a spool or flying lead, which normally has a similar diameter to the flowline and/or the Christmas Tree bore).
Typically, the fluid produced from the subsea oil and gas well is a mixture of hydrocarbons, gases and water. In the cold and high pressure environment which is usually experienced near a subsea oil and gas well, the water may react with gaseous molecules (H2S, CO2 or CH4) to form solid components known as hydrates. The formation of hydrates in subsea equipment can restrict the flow of produced fluid through that equipment. In the worst cases, this can lead to the blockage of the flow through the subsea equipment, as well as damage to pumps and/or valves which may cause them to stop working. This may happen in particular during a production shut down where the fluids cool down into the hydrate formation zone. Several methods have been developed for preventing or managing hydrate formation during the production operation. However, these known methods all have drawbacks.
A known process for attempting to solve the problem of hydrate formation involves replacing the hydrate prone fluid with dead oil or equivalent and deploying a tool called a pig inside the flowline to displace the hydrate prone fluid. The pig is transported along the flowline using fluid pressure. In order to perform this operation, the wellheads to which the pipelines are connected need to be isolated from each pipeline section. The pipelines are then connected to one another so as to form a loop. The pig is then launched from one end of the loop and recovered at the other end of the loop. A disadvantage of this operation is that it is limited to use in pipelines and cannot be utilised to remove hydrates from manifolds or Christmas Trees.
It is also known to use a chemical method for alleviating hydrate deposition at the beginning of production from a subsea oil and gas well. Chemicals such as methanol or methyl ethylene glycol may be injected into subsea equipment, more particularly near a wellhead or manifold, in order to inhibit hydrate formation. However, a drawback of this process is that because hydrate deposits are not porous the chemicals cannot penetrate and break up existing hydrate deposits. Thus, this chemical method does not provide a complete solution, especially when the hydrate is already formed.
One of the parameters that is important in allowing hydrate formation is temperature, ie a low temperature is needed. Thus, in addition to the mechanical and chemical methods described above, a further option is to maintain the temperature inside subsea equipment during production above the temperature required for hydrate formation at the particular operating pressure. This can be done either by insulating and/or heating the subsea equipment. Again, a disadvantage of this operation is that whilst it can be utilised in pipelines it is not practical to use it in areas such as manifolds or Christmas Trees.
Since another important parameter in hydrate formation is pressure, ie a high pressure is required, a further option for limiting the formation of hydrates is to reduce the pressure in the subsea equipment. In order to depressurise (ie reduce the pressure in) subsea equipment, the highly pressurised fluid in that equipment can be extracted through coiled tubing. This process requires the use of a dedicated vessel on the surface of the sea (also known as a topside intervention vessel), from which the coiled tubing is deployed. The pressurised fluid is then transported through the coiled tubing to the vessel. This can be done either under the pressure already in the system, or by the use of a pump. Once the hydrates have been removed from the pipeline, the fluid in the vessel is then reinjected into the pipeline. Drawbacks of this process include that the equipment required to carry it out is expensive and not always available. Also, it can be technically difficult to return the fluid to the flowline. In addition, the fact that pressurised fluid is transported onto a vessel necessitates the detailed consideration of health and safety aspects, especially regarding the storage of flammable oil and gas hydrocarbons on the vessel, which can be time-consuming and costly for the operator.
U.S. Pat. No. 4,589,434 describes an apparatus and method for preventing formation of hydrates in subsea pipelines. This patent is particularly concerned with the prevention of hydrate formation rather than dissociating hydrates which have already been formed. The hydrates in question are those formed as a result of hydrostatic pressure in vertical pipeline sections which are used to transport fluid from a satellite platform to a central platform. The apparatus comprises a reservoir which is in permanent fluid communication with the pipeline. The level of fluid in the reservoir is controlled by pressure of a gas in the reservoir. During normal use, the reservoir is substantially filled with gas. When flow of fluid in the pipeline is stopped, in order to try to prevent formation of hydrates in the vertical pipeline sections as a result of hydrostatic pressure fluid is allowed to flow into the reservoir by reducing the pressure of gas in the reservoir. This is designed to reduce the pressure in the vertical pipeline sections to a level below that required for hydrate formation. The apparatus described in this patent has a number of drawbacks. The reservoir is complex to install and is a heavy load, meaning that installation requires a dedicated vessel with a large crane. A foundation in the form of a pile driven into the sea floor is therefore needed. The large size of the reservoir also means that it is subject to wave motion whilst entering the splash zone. The column of fluid in the reservoir also induces a high hydrostatic pressure. In addition, the fluid is extracted via a riser to the topside.
US patent application publication no 2010/0047022A1 relates to the removal of hydrate plugs from subsea pipelines using depressurisation of the pipeline. However, the procedure involves making a fluid connection via a riser from a topside vessel to the subsea pipeline, as well as storage of the fluid on that vessel. As noted above, such a connection with the fluid from a wellbore means that complex health and safety implications need to be considered when performing such a procedure. A further disadvantage of this method is that the fluid connection to the topside vessel means that there is a column of fluid inducing high hydrostatic pressure at the bottom of the column of fluid.
International publication no WO 2010/151661 A2 describes a system for sampling well production fluids. This system is also mentioned as being used to remove gas hydrate blockages in flow lines. This is understood to be for removing blockages in narrow bore sections of subsea equipment (ie flow lines) by the permanent removal of relatively small quantities of fluid from those lines. Injection of hydrate inhibitors such as methanol is also described. A drawback of this system is that the volume removed is small (5 litres), meaning that it is not efficient enough for hydrate remediation.
International publication no WO 2012/149620 A1 relates to apparatus for depressurising a Christmas Tree in order to prevent hydrate formation or to remove hydrates. The apparatus comprises a pump which extracts fluid from the Christmas Tree and reintroduces that fluid at a different point in the Christmas Tree or other subsea equipment. Disadvantages of this apparatus include that it means that there is a column of fluid inducing high hydrostatic pressure at the bottom of the column of fluid, and that the fluid is extracted via a riser to the topside.
The present invention therefore seeks to address the problem of providing an improved method for reducing the pressure of fluid within subsea equipment, as well as improved apparatus for the remediation of hydrate in subsea equipment, in particular by providing an apparatus that is easier to deploy and recover, is cost effective by using commonly used equipment, and provides improved safety and efficiency. This invention also seeks to provide an apparatus and method for the remediation of hydrate in subsea equipment including pipelines, Christmas Trees and manifolds.